1. Field of the Invention
The present invention relates to a method for forming an interlayer insulating film of a semiconductor device, and more particularly, to a method for forming an interlayer insulating film of a semiconductor device, in which a first oxide film, a Ge boro phospho silicate glass (GeBPSG) film and a second oxide film are sequentially formed over a semiconductor substrate having a topology using a low pressure chemical vapor deposition process, to form the interlayer insulating film.
2. Description of the Prior Art
As semiconductor devices have an increased integration degree, their surface irregularity increases. That is, the topology of such semiconductor devices increases. For this reason, planarization techniques for planarizing a surface having a high topology using an insulating film have been highlighted as an important technique used in the fabrication of highly integrated semiconductor devices. Generally, a BPSG film added with boron (B) and phosphor (P) in a high concentration is used to planarize a surface having a high topology. For the planarization, the BPSG film is thermally treated at a high temperature.
However, the addition of impurities in a high concentration involved in the planarization using the BPSG film results in a degradation in film stability and a formation of crystal defects (BPO.sub.4). Furthermore, the thermal treatment at a high temperature results in a damage of shallow junctions. As a result, there is a limitation on the planarization.
Another planarization method has also been proposed which involves the steps of depositing a GeBPSG film added with Ge as another impurity at a temperature of 350.degree. to 450.degree. C. in accordance with an atmospheric pressure chemical vapor deposition (APCVD) process, and thermally treating the GeBPSG film at a temperature of 750.degree. to 850.degree. C. The addition of Ge results in a formation of GeO.sub.2 which serves to lower the viscous flow temperature of the BPSG film. Accordingly, it is possible to achieve a planarization using the thermal treatment at a temperature of 750.degree. to 850.degree. C.
FIG. 1 is a sectional view illustrating a conventional method for forming an interlayer insulating film.
In accordance with this method, a patterned conductive layer 2, which is to be used as a bit line or word line, is formed on a semiconductor substrate 1. An oxide film 3 is then deposited over the entire exposed surface of the resulting structure in order to prevent moisture and impurities from penetrating into the layer disposed beneath the oxide film when a subsequent process is carried out. In order to planarize the resulting structure, a GeBPSG film 4 is deposited as a planarizing insulating film over the oxide film 3 at a temperature of 350 .degree. to 450.degree. C. The GeBPSG film 4 is then thermally treated at a temperature 750.degree. to 850.degree. C. so that it has a planarized surface.
Where the GeBPSG film 4 deposited at a temperature of 350.degree. to 450.degree. C. is thermally treated at a temperature of 750.degree. to 850.degree. C., as mentioned above, crystal defects (BPO.sub.4) are formed at regions (a) and (c) shown in FIG. 1. As a result, the GeBPSG film 4 has protruded surface portions at the regions (a) and (c). The GeBPSG film 4 is also damaged at a region (b) due to its moisture absorption. As a result, the oxide film 3 is exposed at the region (b).
Since the GeBPSG film exhibits a weakness against moisture and has a porous film structure, it reacts easily with moisture existing in the atmosphere even in a short period after its deposition and before its thermal treatment. As a result, hazes and acid oxide defects are produced. Furthermore, the GeBPSG film may be partially lost, thereby exposing the under layer.
Even after the thermal treatment at a temperature of 750.degree. to 850.degree. C., the moisture absorption property of the GeBPSG film is not improved. Rather, crystal defects (BPO.sub.4) are formed while the thermally-treated GeBPSG film is cooled. Such crystal defects result in a short circuit of a wiring which is subsequently formed. Due to such a degradation, it is impossible to use the conventional method practically.